1. Field of the Invention
This invention relates to a method of manufacturing MOS transistor device and also to the same MOS transistor device.
2. Description of the Related Art
Heretofore, in the manufacture of MOS transistor, the diffusion layer formation step was carried out with main emphasis placed on reducing the resistivity and making the junction shallower. Particularly, with respect to the former there have been developed many techniques inclusive of silicide formation. As for the formation of junction, ion implantation is used mostly for obtaining shallow junction. In this case, the implantation ion dose is usually set to 3 to 5.times.10.sup.15 per cm.sup.2 to hold a low resistivity. This is made so for realizing low resistivity, and there has been no particular discussion about the influence of this on tranasistor. A sole consideration that is paid is to make junction shallow for suppressing the short channel effect.
However, it has been made obvious that the dose of implantation of ions for the diffusion layer formation has influence on transistor. This influence is a "reverse short channel effect" which has been a recent topic of discussion. Specifically, this phenomenon is that the threshold value Vth is reduced with increasing gate length. Concerning this phenomenon, there are many reports, such as H. I. Hanafi et al, "A Model for Anomalous Short-Channel Behavior in Submicron MOSFETs", IEEF Electron Device Letters, Vol. 14, No. 12, 1993, pp. 575-. However, its mechanism is not clear and explained variously.
In addition, it is difficult to reproduce the phenomenon even with simulation equipment which is an important development tool. Therefore, the development has been becoming useless as such. However, the control of the threshold value is a most important subject in low voltage operation. It is thus necessary for this reason as well to suppress the reverse channel effect (see, for instance, Nikkey Microdevices, February 1994, pp. 75).